The present invention relates to magnetic recording medium, such as a magnetic disk, particularly of a quality wherein low noise and high recording density is obtained with flying stability at an ultra low flying height, and the manufacturing method and magnetic storage apparatus making and employing such medium.
Although FIG. 1 relates to a general magnetic recording disk drive of the present invention, it employs features that are broadly conventional, that is a magnetic recording disk 1, spindle 2 to hold and rotate the disk, a magnetic recording head 3 for reading and writing with respect to the disk, servomechanism 4 to position the head, and an electric circuit 5 for driving the above elements. In general, such a disk employs a magnetic recording medium that has an underlayer, a magnetic layer, and a protective layer, all laminated on a substrate. It is important that the read/write characteristics required in the magnetic recording medium provide for durability against the damage of the medium surface by contact with the magnetic head. Especially, it is important to prevent or reduce adsorption or adhesion between the magnetic disk and the magnetic head in the prior art by deposition to produce minute grooves, which are called texture, on the substrate surface, particularly in the circumferential direction or randomly by mechanical polishing using minute abrasive grains, or the like. This type of texture can also be obtained by deposition, particularly by sputtering to produce minute protrusions of the substrate or magnetic layer surface, or both. PTFE (Poly-tetrafluoroethylene) powder may be applied after a protective film is formed as a mask material, then the surface is etched by a dry etching method, and this produces an etching texture of the type mentioned above that has a ruggedness on the protective film surface. Examples of such prior art technology are the following.
Japanese patent Laid-Open No. 61-202324 discloses a mechanical polishing method, called xe2x80x9cTexturexe2x80x9d that forms microgrooves on a substrate, for example in the circumferential direction, to prevent the adsorption or adhesion between a magnetic disk and a magnetic head. Japanese patent Laid-Open No. 60-119635 discloses a sputtering method, called xe2x80x9cDepot Texturexe2x80x9d by forming minute protrusions on a substrate or a magnetic layer to obtain an effect r to that of xe2x80x9cTexturexe2x80x9d. Japanese patent Laid-Open No. 58-53026 discloses a dry etching method, called xe2x80x9cEtching Texturexe2x80x9d, that forms a ruggedness on the protective layer by dry etching using masks made of PTPE powder, etc., to obtain an effect similar to that of xe2x80x9cTexturexe2x80x9d.
It is an object of the present invention to provide a magnetic recording medium, an apparatus employing such medium, and a method to produce such medium, wherein roughness of the magnetic layer is smaller than the roughness of the substrate, in the field or area where writing and reading is carried out by a magnetic recording head. Therefore, a magnetic recording medium and magnetic recording apparatus is obtained with high recording density and high reliability, by the method and structure of the present invention that reduces media noise and provides flying stability of the magnetic recording head at ultra low flying height, because of the reduced ruggedness of one or more of the underlayer, magnetic layer and protective layer, with respect to the roughness of the prior art.
An object of the present invention is to reduce or prevent adhesion of the magnetic head to the magnetic disk for increased reliability for assurance-proof of the contact start-stop (referred to as CS/S), which according to the above-mentioned prior art has previously involved increasing roughness and therefore reducing flying height in the prior art and reducing recording density in the prior art. Therefore, the present invention has simultaneously two objects that have heretofore been mutually exclusive of each other.
The inventors"" analysis of the prior art problems and causes are a part of the present invention.
At the present time, high recording density involves a flying height of the magnetic head that is below 20 nm. As shown by conventional evaluation methods such as CS/S, durability and adhesion (friction) became difficult to obtain with respect to proof stress of the data plane because of irregularity in contacting between the head and medium. This was especially true with the adoption of LZT (Laser Zone Texture), wherein the CS/S zone and data plane or data field are distinguished, and this becomes more important with respect to reliability of the data plane. In general, damage from impact was lessened and adhesion of the magnetic head prevented by the prior art adoption of a lubricant and by adding minute grooves, previously referred to as texture or TEX, by the conventional machining of the data plane, which has the same surface as the CS/S zone. According to such prior art method, the TEX processing itself increases surface roughness of the substrate, and as the surface roughness of the substrate increases, necessarily the flying height of the magnetic head inevitably rises. Therefore, levitation start height in the medium plane of the magnetic head, Hto (the head takeoff height), also rises. Therefore, with such prior art technology, it is not possible to make the flying height of the magnetic head less than 20 nm or less, which is necessary to obtain ultra low flying height and high density. To obtain the flying height of 20 nm or less, it is necessary that the starting height of the levitation, that is Hto, be 10 nm or less, when dispersion of the buoyancy of the head and assembly accuracy of the drive are considered.
With respect to the conventional technology, it is shown in FIG. 2, that the surface roughness of underlayers 8, 9, a magnetic film 7 and a protective film 6, which are laminated on a substrate 10, have a surface roughness dependent upon the surface roughness of the substrate. In general, the inventors"" analysis of the prior art shows that the surface roughness of the finally completed magnetic recording medium is increased with respect to the surface roughness of the substrate.
FIG. 3 shows a substrate surface (a) obtained by texture-processing using abrasive grain and formed of 30 nm of Cr deposited on the substrate surface (b), and Table 1 shows data for the surfaces as measured by an atomic force microscope (AFM) for comparison.
A numerical value, as an index to surface roughness, is obtained by the following formula for center line average roughness of the roughness curve: Ra,                     Ra        =                              1            /            L                    ⁢                      xe2x80x83                    ⁢                                    ∫              0              L                        ⁢                                          "LeftBracketingBar"                                  f                  ⁡                                      (                    x                    )                                                  "RightBracketingBar"                            ⁢                              ⅆ                x                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
L is the length of a roughness curve (distance of actual measurement), the roughness curve (data of actual roughness was shown as a curve, and Ra is obtained by the integral of this absolute value): y=f(x).
Ra: center line average roughness shown in the above equation.
Rp: The largest center line height (largest interval with the center line in the summit in the roughness curve). Rmax; Maximum height above rail level in the roughness curve (peak to peak). The index to this surface roughness has generally been adopted. The AFN used was a NANO-SCOPE III made by DI (Digital Instruments, Inc.). As it is clear from FIG. 3 and Table 1, the substrate surface by the mechanical TEX working method has actually considerable abnormal points (protrusions), and this is shown in maximum height above rail level, Rmax. As shown in Table 1, Rmax of the underlayer shows a larger value than the substrate, because the surface which formed the underlayer reflects the surface of the substrate, and there is the growth of the protrusion of Cr by the abnormal growth which will occur in epitaxial growth of Cr at the abnormal point of the substrate. Therefore, it is not aft possible to obtain a lower flying height of the magnetic head than permitted by the roughness of the surface on magnetic recording medium produced by this prior art; this is the main cause of the crash, and the difficulty in obtaining the high recording density and R/W characteristics. Also, there is the rising probability of the magnetic head contacting the abnormal convex in the slide-proof surface.
In general, a Nixe2x80x94P film is formed on the Al alloy substrate as a nonmagnetic substrate material applied by plating at about 10 xcexcm and processing by machining the surface to a mirror surface, and then cleaning and drying after texture-processing, which used abrasive grains. Then the underlayer, magnetic layer and protective layer are formed in order in the vacuum film formation equipment, and that produces the magnetic recording medium. A hardness and Young modulus of the Ni/Alxe2x80x94P substrate surface used here are respectively 7, 42 Gpa and 147/Gpa (Nano Instruments, Inc.) (measurement needle: Berkovich type tip, diameterxe2x80x9d R100 nm, material: Diamond), when it is measured by the micro penetrometer, (at the 60 nm indentation depth).
In this prior art it is easy to deform plastically the surface laminated metal layer, even if the hardness of the metal layer is high to some degree, as the whole thickness of the metal layer is laminated to about 30xcx9c100 nm; therefore it is not possible to endure the contact impact of the magnetic head, because of the Nixe2x80x94P softness and tendency of the deformation.
Therefore from the above inventors"" analysis of the prior art, it is seen that the prior art texturing, while reducing the adhesion tendency between the head and medium, also increases the probability of head crash and/or significantly increases flying height or at least prevents ultra low flying height, while at the same time having a tendency to provide an easily deformed surface that will not have high reliability.
The inventors I analysis of the prior art and problems of the prior art as well as the causes of such problems are a part of the present invention.
While the prior art has addressed increased smoothness of the substrate surface and especially the substrate surface in the data area or writing and reading field, as a way of reducing flying height of the magnetic head to obtain high recording density, which is a requirement for future technology, such increased smoothness according to the prior art has increased the probability of head crash or contact at high speed. The inventors"" analysis has shown that to date the prior art has not been able to accomplish the above objects without at the same time degrading other desired characteristics of other objects, that is the prior art has been unable to simultaneously improve impact resistance and move towards the future requirements of high density recording. It is an object of the present invention to overcome such problems.
It is an object of the present invention to reduce medium noise by reducing turbulence of the magnetic domain by providing flatter surface roughness of the magnetic layer and/or the protective layer relative to the surface roughness of the substrate, which cannot be obtained with the conventional magnetic recording medium, while at the same time obtaining high density recording f or the magnetic recording medium and high density storage by improving the levitation stability of the magnetic head and lowering the absolute flying height of the magnetic head.
According to the present invention, two-dimensional square mean square root roughness Rq of the magnetic layer is less than two-dimensional square mean square root roughness Rq of the substrate surface, and thereby the medium noise decreases in the data plane or field, which is the field of reading and writing by the magnetic head on the magnetic recording medium, particularly a medium having an underlayer, a magnetic layer and a protective layer. Further, according to the present invention, the two-dimensional square mean square root roughness Rq of the magnetic layer and protective layer is smaller than at least the two-dimensional square mean square root roughness Rq of the substrate surface, and at the same time the medium noise in the data plane, which is the field of reading and writing by the magnetic head on the magnetic recording medium, is reduced while reducing the absolute flying height of the magnetic head for improved levitation stability. As a result, high recording density of the magnetic recording medium is obtained.
Particularly, Rq of the substrate is 5 nm or less and Rq of the magnetic layer and protective layer are 3 nm or less, so that there is reduction of the medium noise and reduction of the flying height of the magnetic head to 20 nm or less to obtain levitation stability and ultra low flying height as well as high density recording resulting therefrom.
To obtain the flatter surface roughness according to the present invention, the surfaces, such as surfaces of the underlayer and substrate are exposed to gas ions of a plasma within a vacuum to prevent or reduce protrusions of the surface by abnormal growth of the layers to thereby reduce the surface roughness so that the smooth film is obtained without the abnormal growth protrusions.
The results of conducting such surface treatment according to the present invention are shown in FIG. 4. As an underlayer, a Cr film was formed to a thickness of 30 nm on a TEX processed substrate. Ionized nitrogen gas, N2, was used with parallel plate etching equipment by RF plasma to strike the substrate surface, and analysis of how the surface changed during processing time was carried out by AFM, ESCA. As a result, it was proven that the surface roughness was reduced with processing time, as shown in FIG. 4(a). ESCA (Electron Spectroscopy for Chemical Analysis) employs X-ray photoelectron spectroscopy analytic equipment to determine surface roughness quantitatively by photoelectrons using an X-ray source; this is a base for the peculiar, scientific and material combination, and a base for measuring the energy. The measurement X-ray emission conditions were: Monochromatization X-ray (ALK xcex1) emitted beam of beam diameter of 200 xcexcmxe2x88x9244 W to an Al target as an excitation source.
The take out angle: 75 degrees.
The analysis region: the point analysis.
The pulse energy: 29.35 ev.
The energy spread: 0.125 eV.
The apparatus used was Quantum 2000, made by ULVAC PHI, Inc.: it is a Scanning ESCA Microprobe System.
Nitrogen, N, was detected at the Cr film exposed with the N2, as a result of ESCA analysis of the surface, as shown in the following Table 2. It is shown that both the peak of C, which seems to be the hydrocarbon, and the peak of O decrease; and this result shows that the surface was purified. In Table 2, the electron orbits are shown by 2 p for Cr and by 1 s for C, N and O.
FIG. 4(b) shows that hardness increases as the processing time increases, with surface hardness being measured by a minute penetrometer made by Nano Instruments, Inc. As a result of such examining, it is determined that in addition to using RF etching equipment, the present invention further contemplates that such ionized gas exposure may be obtained by the use of ion guns, plasma guns, etc., to obtain a similar effect.
The above results show a surface flattening of the substrate and/or the underlayer by processing with gas ions, particularly nitrogen gas ions, in the form of a plasma accelerated into the surface to produce migration of only the pole surface of the metal layer by a rise in the temperature of the pole surface of a film. The atomic radius of the N atom, which atomic radius is 0.52 xc3x85 (0.053 nm), is much smaller than the size of the metallic protrusion radius that is about 1.25 xc3x85. Therefore, it appears that the N atom penetrates into the film without having the physical etching action, and as a result the metal appears to migrate as most of the ionized gas energy is converted into thermal energy, which results in a flattening of the surface roughness. While some etching may occur, substantial etching does not and it appears that the important factor is this increased heat that produces a melting and migration of the metal protuberances for a general flattening, and therefore a highly micro amorphous structure is obtained because the surface of the metal film is changed to a nitride. Therefore, the crystal grain of film laminated at the upper part of the metal film becomes smaller, and as a result, roughness of the magnetic layer surface and protective layer surface becomes smaller as compared with the roughness of the substrate and as compared to such surface without such processing. By this reduction of surface roughness of the magnetic layer and of the protective layer, high density recording is obtained by reduction of medium noise and improvement of the magnetic head levitation stability in the ultra low levitation region is achieved.